RUMBLE SEAT / DAN NEIL

All the pretty horses

There are 604 of 'em in Mercedes' speedy SL65. Look closely and see an era pass by.

There is something distinctly epochal about the Mercedes-Benz SL65 AMG, the twin-turbocharged, 12-cylinder, 604-horsepower version of the company's quantum-luxury roadster. More powerful than the Ford GT and quicker than a Ferrari 360, the SL65 -- an SL600 with gene-doping enhancement by AMG, the company's tuner division -- feels like a final, conflict-ending fusillade in the decade-long horsepower wars.

What comes after a car like this? More horsepower? I don't think so. While it's certainly possible to wring more horsepower out of an emissions-legal gasoline engine, such pursuits grow increasingly expensive and irrelevant, if not vain and silly. The SL65 is less than a second slower to 60 mph than a 650-horsepower Ferrari Enzo (3.3 seconds) -- and that single second, that fig leaf of numerical superiority, costs about a half-million dollars more (assuming you could buy an Enzo at anything like its $680,000 MSRP).

The long-awaited Bugatti Veyron, a million-euro hyper-car built under the auspices of parent company VW Group, promises nearly 1,000 hp out of its quad-turbo W16 engine. But it looks to be a rather feckless enterprise; with its complex drivetrain (seven speeds, multiple differentials and all-wheel drive) the 4,300-pound car will not be quicker than the 3,000-pound Enzo, though if it manages to stay on the ground its top speed will probably be higher.

The horsepower wars are, at base, psychological warfare. It's purely academic if the 400-hp Cadillac CTS-V is faster than the 500-hp BMW M5 (it isn't), since both cars are horridly fast enough. You may indulge your pet theories about men and their sexual compensations and I, for one, wouldn't argue -- though I might cross my legs.

Extreme sports cars justify their ludicrous cost only by a few tenths of a second measured by some car magazine, and yet here the field seems to be reaching a natural equilibrium -- they can all make the green-light dash in around 4 seconds; they can all turn a quarter-mile in about 12 seconds; and they can all cruise at 200 mph if they happen to be crossing the U.A.E. with one of the emir's sons on board. In this category, you will find cars like the Porsche Carrera GT, Ford GT, Lamborghini Murcielago (3.6 seconds); and a handful of rare isotopes with names like Koenigsegg, Pagani and Mosler.

So where is the value of these cars when an SL65 can stalk them? All of these ground-bound rockets are purpose-built from lightweight, hard-to-spell metal alloys and carbon composites. The all-steel SL65 weighs 4,473 pounds and is, in its bones, the same chichi chariot seen tooling around Bel-Air, a courtesan cruiser. Its long-as-your-arm list of standard features includes a retractable hardtop, five-speed automatic transmission, air-conditioned and heated seats with massage function, surround-sound stereo, DVD navigation, even a climate-controlled glove box.

The SL65 goes from a standstill to 60 mph in 4.2 seconds, clears the quarter-mile traps in 12 seconds and only then does it find its stride in a jubilant, wind-cleaving roar. In the time it takes to read aloud this paragraph, the car will go from zero to its electronically limited top speed of 155 mph.

What's that feel like? Ask the guy in the funny helmet who gets shot out of a cannon.

You can almost feel sorry for exotic cars. Stuffing more horses in the corral won't help.

Among the limiting factors: street tires. The SL65's bi-turbo V12 is a veritable Vesuvius of torque -- 738 pound-feet between 2,000 and 4,000 rpm -- yet the trick is converting all that driveshaft twist to acceleration. If you nail the throttle in the SL65 (traction control turned off) the rear wheels simply boil off a few hundred dollars of exotic Italian rubber (even with its relatively high rear-end ratio of 2.65:1, which favors top-end speed over hole-shot quickness). You could increase the width of the tire or reduce the water-channeling grooves -- increasing the overall "contact patch" -- but you only make the tires louder, less compliant in ride quality and less secure in wet conditions. You could make the rubber compound softer and more adhesive but then the tires would wear rapidly.

All-wheel-drive divvies up engine torque among four wheels, but given the additional driveline weight and complication a car may actually go slower, as in the case of the Veyron. Also, highly strung engines require more cooling airflow, which complicates high-speed stability. Monster torque at the output shaft requires a transmission that can handle it. Again, more weight, more complication. These are the sorts of where-the-lines-cross considerations automotive engineers make all the time.

Another limiting factor: durability. There are import tuners out there who are getting 500 horsepower out of their turbocharged four-cylinder Mitsubishi Evos -- once, maybe twice, before they grenade in a shower of exotic particles.